Consider a regional power system having a strong internal transmission system transmitting power to another strong regional system on relatively weak Interties. Such a regional power system may experience issues with stability during disturbances, such as short circuits, loss of generation, loss of load, loss of one of the Interties, or any combination thereof. Prevalent practice to the solution of these issues is to include more Interties, increase the voltage to higher voltage levels (such as extra high voltage (EHV) levels or ultra high voltage (UHV) levels), or both. Another approach for better power system stability is to employ protection relays with high operation speed.
Travelling wave protection is one approach for super-high speed protection. There are different types of travelling wave protections, for example, travelling wave pilot protection based on directional comparison, travelling wave current differential protection, travelling wave protection based on distance measurements, etc.
In general terms, directional pilot protection enables a practical and reliable mechanism for travelling wave protection. It only needs a small bandwidth channel to transmit binary information between terminals at end points of a transmission line. Directional pilot protection can detect the fault direction based on local measurements, which can block the mal-trip on external fault in reverse direction even with wrong communication.
Travelling wave protection based on directional comparison has e.g., been presented in “Ultra High Speed Relay for EHV/UHV Transmission Lines'—Development, Design and Application” by M. Chamia and S. Liberman in IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 6, November/December 1978. In general terms, in such protection system, the voltage and current at both ends of the protected line are measured. The direction of the local voltage and current measurements is detected. A trip decision is made based on a comparison of the directions of both ends.
In more detail, in travelling wave protection based on directional comparison the polarities of the first wave fronts of local voltage and current are compared. If they are the same, a backward fault has occurred. If they are reverse, a forward fault has occurred. Then the protection relays at the two terminals will transmit the fault direction to the other terminal. If both directions are forward directions, an internal fault has occurred. Otherwise, an external fault has occurred. Its basic principle is shown in FIG. 2.
One issue with the above disclosed travelling wave protection based on directional comparison is the presence of harmonics. A low pass filter and some band stop filters are used in above disclosed travelling wave protection based on directional comparison to filter the travelling wave components. In modern power system, with more and more flexible alternating current transmission systems (FACTS), high-voltage direct current (HVDC), and other harmonic resource, the harmonics are complex and may be sub-harmonics. It may be difficult, or even impossible, to remove all the harmonics by pre-setting band stop filters. As a result, the harmonics may be treated as travelling wave, which may lead to mal-trips in some cases when such false travelling waves indicate wrong forward fault directions from both ends.
Capacitive Voltage Transformers (CVTs) are widely used in EHV/UHV systems as voltage signal measurement sensors. But CVT does not have a flat frequency response, especially for high frequencies. This leads to measurement errors, which can be big, for fault transient detection. For current travelling waves, it may be easy to avoid harmonic impact by using high-frequency current. But for voltage travelling waves, because of the generally bad frequency response of CVTs, only relatively low frequency value are available (e.g., <1 kHz). This means that voltage may be influenced by harmonics more seriously. As a result, the polarity of voltage travelling wave detected may be wrong in some cases. Thereby, reverse faults may be treated as forward faults, which may lead to a mal-trip in some cases.
CN102122815 A provides a traveling wave direction pilot protection method. The directional detection is based on a current travelling wave (including high frequency components) and a fundamental frequency voltage (50 Hz or 60 Hz). This method can avoid the influence from voltage harmonics by using fundamental frequency voltage. But the usage of low frequency components also slows down the operate speed. In addition, this method is based on directional comparison. A possible erroneous detection of voltage polarity caused by other reasons except the harmonics may still bring a risk of mal-trip.
However, there is still a need for an improved protection of a transmission line, for example to provide improved travelling wave protection which can reduce the risk of mal-trip even when the detection of voltage polarity is wrong.